Conventional systems for supplying conditioned air from a central source to a one or more climate controlled spaces, such as offices, classrooms, and other areas that may from time to time be occupied, commonly use variable air volume (VAV) distribution. Such VAV systems include a variable capacity refrigerant vapor compression system and a variable speed fan. The refrigerant vapor compression system includes a compressor that circulates cold refrigerant through a heat exchanger coil in heat exchange relationship with air passing over the coil to cool the air to be supplied to the climate controlled space to a preset air supply temperature. The speed of the variable speed fan is varied between its lower and upper speed limits to vary the volume of air supplied through a ductwork system to the climate controlled space. Typically, the speed of the fan is varied in response to duct air pressure to maintain a desired static pressure head to ensure adequate distribution of the air through ducts to a plurality of areas within the climate controlled space. In conventional VAV systems, air terminals equipped with dampers controlled by actuators responsive to room temperature are associated with the supply air registers. The actuator selectively positions its associated damper at a desired position between fully open and minimum ventilation position to control the amount of cold air from the supply duct into the room.
In conventional or mixed air distribution systems commonly associated with conventional VAV systems, the cold air being supplied to the rooms and other areas within the climate controlled space is admitted through registers disposed in or near the ceiling of those areas, while return air is drawn from those areas through return registers in or near the ceiling thereof.
The same HVAC equipment can also be used in what is often referred to as a single zone VAV system. In the single zone VAV system the refrigeration capacity is controlled to supply constant temperature air and the volume of air is varied to control the temperature of the room. The same equipment can also be used in a thermal displacement ventilation system. In thermal displacement ventilation air distribution systems, the cold air being supplied to the climate controlled space is admitted through registers disposed in or near the floor of the space, while return air is drawn from the space through return registers in or near the ceiling of the space.
In operation of such VAV systems, the ability to optimally control the comfort temperature and also humidity within the climate controlled space may be limited at times of high cooling demand, such as during periods of high occupancy and/or high outdoor temperature and humidity, because air supply temperature and fan speed are controlled to a fixed setpoint. Also with fixed setpoint control, the air may not be optimally dehumidified resulting in a high humidity level in the space or the expenditure of excess energy to overly dehumidify the space.